Chlorite schist is a regional metamorphic rock with chlorite as its main mineral component, and it has a schistose structure and is water-disintegrable and easy to be softened. Since it belongs to a typical soft rock with low bearing capacity and strong deformation capability, even if it is exposed to be a relatively integral surrounding rock in the early period of tunnel excavation, it will be subject to a large deformation in a short period of time and can hardly be self-stable. If the exposed surrounding rock has poorer integrity, its primary stability can hardly be maintained if direct excavation is carried out, and even a serious collapse will occur in the excavation process, which brings huge difficulties to tunnel construction. Excavation will cause a large range of disturbance to the surrounding rock especially for large-span tunnel such as deep single-hole highway tunnel with three lanes. What's worse, when the ground stress is high, the occurrence of large extrusion deformation is inevitable. If inappropriate measures are adopted, it is easy to cause the damages such as the clearance intrusion of tunnel support deformation, the cracking and spalling of shotcrete, the distortion and fracture of steel frame, the cracking and chipping or even the collapse of secondary lining, and the sinking of side wall, which delays the construction and greatly increases the cost, and the safety is out of control.
Therefore, it is particularly important to reasonably predict the large deformation grade of large-span tunnel in chlorite schist stratum and then adopt corresponding control measures so as to avoid the clearance intrusion caused by tunnel deformation and to ensure the safety of tunnel construction and the stability of the support structure, and the cost is controllable. However, there is no current classification scheme for the large deformation of large-span tunnel in chlorite schist stratum and its corresponding control measures. Therefore, it is necessary to propose the classification scheme for the large deformation and its corresponding control measures on the basis of understanding the physical and mechanical properties of chlorite schist under the different conditions of surrounding rocks by means of the detailed geological investigation, field practice and experimental research.